1. Field of the Invention
The present invention relates to a developing unit. More specifically, the present invention relates to a developing unit that provides developer to the surface of a developer support member installed in an image forming device.
2. Background Information
An electrophotographic image forming device such as a laser printer or a copying machine has a photosensitive drum and a developing unit that provides toner to the surface of the photosensitive drum. In general, the developing device includes a developing roller that provides toner to the photosensitive drum, an agitation transport member that agitates the developer and simultaneously transports the developer in the axial direction and provides the same to the developing roller, and a housing which includes the developing unit and the agitation transport member in the interior thereof. The agitation transport member is normally comprised of a rotary shaft, and a helical blade that is formed on the outer circumference of the rotary shaft.
An example of the conventional developing unit is disclosed in Japan Patent Application Publication JP-A-09-160360. The circulation agitation method is used in this developing unit, and developer is circulated and agitated by two agitation transport members that are arranged parallel to each other. In addition, a toner replenishment hole is formed in a region of the housing located between both ends thereof in the axial direction of the agitation transport members. Furthermore, an external diameter “R” of one region of a blade formed on one agitation transport member which faces this toner replenishment hole, is formed to be larger than an external diameter “r” of another region of the blade formed on the agitation transport member. Thus, the replenished toner can be efficiently agitated into the developer.
However, a gap “g” formed between the agitation transport member and the housing in regions except for the vicinity of the toner replenishment hole has to be formed to have a length that is at least half of the difference between the external diameter “R” and the external diameter “r” of the blade (i.e., at least (R−r)/2). Therefore, the gap between the blade and the housing in regions except for the vicinity of the toner replenishment hole will inevitably be larger. Thus, agitation/transport efficiency is significantly reduced.
On the other hand, the gap “G” between the agitation transport member and the housing in the vicinity of the toner replenishment hole cannot be set smaller than this value, in order to avoid contact between the agitation transport member and the housing caused by the deflection of the rotary wobbling of the agitation transport member in the radial direction.
A developing unit using a conventional circulation agitation method has been proposed, in which one of the agitation transport members is formed to be longer than the other agitation transport member in the axial direction. Here, a portion of the longer agitation transport member is used as a toner supply region containing externally replenished toner. In this unit, toner is transported from the toner replenishment region to the toner supply region located downstream of the toner replenishment region, and is circulated and transported between the longer agitation transport member and the other agitation transport member.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved developing unit. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.